Research

Inspiration from advanced structures in nature

Demystify the molecular structure of animal silks and predict their mechanical properties

Animal silks with semi-crystalline molecular structures are spun from a spinneret specially evolved for producing fibres for structural purposes. There are a great variety of animal silks in nature, among which the two most famous are spider dragline silk and silkworm’s cocoon silk. Spider dragline silk, as its name suggests, is the thread that spiders often drag behind as a life line, which displays great strength, extensibility and hence toughness. The highest toughness of spider silk is reported to be 350MJ/m3 [1], much greater than that of carbon fibre and Kevlar. Compared to the “rare” and expensive spider silk, silkworm silk is produced in tons every year and has been utilized as a superior textile fibre for many centuries. The physical properties of silk, for example, its lustre, breathiness and good thermal conductivity, are more impressive than plant fibres and synthetic fibres. However, the structural origin of these unique properties of animal silks has been disputed over the years, and it remains a challenge to establish the structure-property relationships for such fibres.

Dr. Guan and collaborators used a classical experimental technique, Dynamic Mechanical Thermal Analysis/DMTA, and successfully obtained the thermal-mechanical property spectra for several kinds of animal silks, including a few microns-thin spider dragline silk. The glass transition behaviours of these animal silks were analysed and compared quantitatively for the first time.

In general, the semi-crystalline structure of animal silks can be described as an order-disorder two phase model, defined by three structural parameters: ordered/disordered structural fraction, fdis; the glass transition temperature of the disordered structure, Tg; and the melting temperature of the ordered structure, Tm. Dr. Guan’s new study further developed the Group Interaction Modelling and introduced specific amount of hydrogen bonding energy into the cohesive energy of the interacting group, to provide a quantitative relationship between the hydrogen bonding density and the glass transition temperature in the disordered structure of silk. Furthermore, the disordered molecular structure in silk can be determined from the DMTA spectrum. This work [2], published in Soft Matter by the Royal Society of Chemistry, elucidates on the role of hydrogen bonding in enhancing the thermal and mechanical properties of biopolymers.

 

Strong and tough wild cocoons to inspire new designs for synthetic fibre composites

Cocoons are protective housings fabricated by insects of the Order Lepidoptera before caterpillars change to moths in a process called metamorphosis. These housings make sure the pupa inside can live through the long period of metamorphosis. In the North of China, a wild silkworm species, Anthearaea pernyi, lives in the oak tree forests. The cocoons from this silkworm species are naturally required to possess air tightness and mechanical robustness to protect pupa from frigid environment and ravenous predators. In the South of China, another silkworm species, Bombyx mori, is cultured in houses of farmers for two seasons a year, and the end product of this agricultural activity is the silk fibre for textile uses. Both types of cocoons contain silk fibre and a protein glue to form a 3D woven structure. However, the functionality of the two cocoons may suggest different property profiles of the two cocoons- the survival game of nature demands much greater mechanical performance for the wild silkworm cocoon.

Evidently, the tensile mechanical strength of the wild cocoon composite is 2 times greater than that of the domestic cocoon composite, and the energy absorbed during tensile deformation is 5 times greater for the wild cocoon composite (refer to the Figure). This fact leads Assoc. Prof. Juan Guan and Prof. Jun Xu to study in detail the secrets behind the superior mechanical performance of wild cocoon composites. They found that the microstructure and the material properties are the two contributions to the strong and tough mechanical performance of the wild cocoon composite. The microstructural “design” including a very thin layer of glue and the ribbon-flat silk fibres ensures the robustness of the fibre network. On top of the microstructure, the strong and tough tensile mechanical properties of the wild silk fibres contribute to the toughened mechanical behaviour and enable a tough failure mode for the composites. The finite element modelling (FEM) led by Prof. Jun Xu proved further that both the robust fibre network and the tough mechanical properties were required for strong and tough fibre composites. This study sets an example of combining experiments with fundamental understanding, and puts forward a simple model of fibre composites to simulate the complex interactions between the fibre and the glue binder in the composite.

Since 2014, Assoc. Prof. Juan Guan and Prof. Jun Xu have established a strong teamwork with commitment towards mutual interest in understanding the behaviors of composite materials and structures. This joint research effort contributed from both sides shows a significant advantage of interdisciplinary collaboration across Schools within the International Research Institute for Multidisciplinary Science.

 

 

 

Fig. (A) (B) Photo images of domestic B. mori and wild A. pernyi cocoons; (C) Tensile stress-strain curves of the two cocoon composites

 

Juan Guan, associate professor, school of material science and engineering, Beihang University, E-mail: juan.guan@buaa.edu.cn

 

 

References

[1] https://en.wikipedia.org/wiki/Spider_silk

[2] Guan, J.; Wang, Y.; Mortimer, B.; Holland, C.; Shao, Z.; Porter, D.; Vollrath, F. Glass transitions in native silk fibres studied by dynamic mechanical thermal analysis. Soft Matter 2016, 12 (27), 5926-36.

[3] Xu, J.; Zhang, W.; Gao, X.; Meng, W.; Guan, J. Strain Rate and Anisotropic Microstructure Dependent Mechanical Behaviors of Silkworm Cocoon Shells. PLoS One 2016, 11 (3), e0149931.

[4] Guan, J.; Zhu, W.; Liu, B.; Yang, K.; Vollrath, F.; Xu, J. Comparing the microstructure and mechanical properties of Bombyx mori and Antheraea pernyi cocoon composites. Acta Biomater. 2017, 47, 60-70.